The present technology relates to an image forming apparatus that performs electrophotographic image forming such as a copier or a printer, and more particularly to an image forming apparatus having a toner recycling function for reusing toner collected from the surface of an image supporting body.
With an electrophotographic image forming process implemented in an image forming apparatus such as a copier or a printer, an electrostatic latent image formed on the surface of a photosensitive drum (image supporting body) is developed into a visual image (hereinafter, “toner image”) by a powder developer (hereinafter, “toner”) supplied from a developing device. This toner image is transferred to paper (including recording media such as OHP etc.) in a transfer process. At this time, part of the toner remains on the surface of the photosensitive drum and is not transferred to paper.
Because the photosensitive drum is used repeatedly in image forming process, leaving toner that remains on the surface of the photosensitive drum (residual toner) deteriorates image quality the next time image forming process is performed. The image forming apparatus thus eliminates and discards residual toner on the surface of the photosensitive drum using a cleaning device.
However, running costs rise due to increased wastage of toner as a result of residual toner being discarded. In view of this, there are conventional image forming apparatuses using a two-component developer composed of a toner and a carrier that have a so-called recycling mechanism whereby residual toner eliminated from the surface of the photosensitive drum by a cleaning device as shown in FIG. 1 is collected in a developing device and reused for suppressing toner wastage.
The image forming apparatus shown in FIG. 1 has a photosensitive drum 51, and further includes an image forming portion 50 that has a charger 52, a developing device 54, a transferer 55, a cleaning device 57, a neutralizer 58, and the like disposed in this order along the circumferential surface of the photosensitive drum 51. The image forming portion 50 also includes a recycling mechanism 59 that transports residual toner from the cleaning device 57 to the developing device 54. The cleaning device 57 includes a blade 57A that contacts the surface of the photosensitive drum 51, and scrapes off and collects residual toner adhering to the surface of the photosensitive drum 51, after the toner image has been transferred to paper at a transfer location between the photosensitive drum 51 and the transferer 55.
The recycling mechanism 59 has a screw conveyor mechanism, and transports residual toner collected in the cleaning device 57 as recycled toner to the inside of the developing device 54 at a prescribed timing.
To evenly maintain the toner density of images formed on paper in an electrophotographic image forming process that uses a two-component developer, sufficient toner necessary to visualize the electrostatic latent image needs to be held in the developing device.
For this reason, image forming apparatuses adopt a configuration in which the amount of toner held in the developing device is detected based on the detection signal (toner density) of a toner density sensor that uses a magnetic permeability sensor, for example, and toner is supplied to the developing device based on this detection result.
The magnetic permeability sensor detects the magnetic permeability of the two-component developer in the developing device. A change in the magnetic permeability indicates a change in the toner consumed from the two-component developer. The magnetic permeability of the toner is low, while the magnetic permeability of the carrier is high. Also, the amount of carrier hardly changes. Thus, the magnetic permeability of the two-component developer decreases the greater the amount of toner in the developing device.
However, in the case of a direct transfer method in which the toner image is transferred to the paper touching the surface of the photosensitive drum, paper dust (paper fibers, paper additives, etc.) adhering to the paper shifts to the surface of the photosensitive drum during the transfer. Paper dust, which has a low magnetic permeability, is thus eliminated by the cleaning device together with the residual toner, and gets mixed in the recycled toner, thereby affecting the precision at which toner density is detected by the toner density sensor.
This results in either too much or too little toner being supplied to the developing device, making it impossible to properly maintain image density. In particular, much of the paper dust once collected is continually transferred to paper and circulated around the developing device, the photosensitive drum and the recycling mechanism. Thus, when residual toner is continuously recycled by the recycling mechanism, the effect on the precision at which toner density is detected gradually increases with the gradual increase in the density of paper dust in the developing device.
In the developing process for visualizing an electrostatic latent image formed on the surface of the photosensitive drum, paper dust with toner adhered to the periphery thereof shifts from the developing device to the surface of the photosensitive drum together with the toner. As a result, in the transfer process for transferring a toner image formed on the surface of the photosensitive drum to paper, some of the paper dust with toner adhered to the periphery thereof shifts to the surface of the paper, while the rest remains on the surface of the photosensitive drum and is again collected by the cleaning device together with the residual toner. Further, when transferring the toner image to paper, some of the paper dust adhering to the paper also newly adheres to the surface of the photosensitive drum and is collected. Here, it is thought that the amount of paper dust gradually builds up because the amount of paper dust that attaches to the photosensitive drum from the paper is greater than the paper dust transferred together with the toner from the photosensitive drum to the paper.
Also, when paper dust with toner adhered to the periphery thereof adheres to the surface (white background) of paper in the transfer process, undesired dots are formed on the paper, reducing image quality.
In view of this, with an image forming apparatus disclosed in JP6-308828A, for example, for solving these problems, an oscillatable mesh paper dust trapping member is disposed arbitrarily on the collection path of the residual toner, and only reusable toner is separated out by eliminating paper dust included in the collected toner.
An image forming apparatus disclosed in JP H10-214011A includes a sorting roller charged to a negative potential which is a sorting means that makes use of differences in the amount of charge, and collects only adequate toner of a suitable particle size that is charged to an adequate positive charge from the surface of the photosensitive drum. Note that a cleaning portion collects the remaining inadequate toner.
Further, an image forming apparatus disclosed in JP 2005-3783A has a means for measuring the amount of impurities in residual toner on the toner recycling path, and selects whether to circulate the residual toner to the developing device or discard the residual toner according to the measurement result.
Note that the paper is broadly divided into medium and high quality, and that a recycled paper classification also exists depending on the manufacturing method. There is a process for removing a component called “lignin” included in tree fibers that is like pine resin at the stage of manufacturing pulp fibers, with lignin being removed from “high quality paper” but remaining in “medium quality paper” (plain paper). High quality paper is white paper such as copy paper and print paper, while medium quality paper is used in newspapers, magazines, and cardboard. Medium quality paper is of poor quality, and is almost never used for image forming in electrophotographic image forming apparatuses such as copiers or printers, which have strict requirements regarding paper quality.
In contrast, the use of recycled paper made from recycled high quality paper for image forming in electrophotographic image forming apparatuses has recently been increasing in consideration of the global environment. Recycled paper is a mixture of different paper fibers that is discolored with printing ink and whose fibers are unavoidably degraded through repeated use. For this reason, the quality of paper whose raw material is recycled pulp (recycled paper) drops in comparison to paper made with new raw materials from timber (virgin pulp). Thus, the amount of paper dust (mainly paper fibers) produced with recycled paper is much greater than high quality paper.
However, a configuration in which recycled toner is collected so as not to include paper dust as described above requires expensive apparatuses such as paper dust trapping members, sorting rollers, sensors for detecting the amount of paper dust such as paper dust in toner or masses of aggregate toner and the like, thereby giving rise to cost hikes.